chapter 4 Cell Structure

Chapter 4: Cell Structure

Overview

• Cells are fundamental units of life, organized compartments bounded by plasma membranes.

• The cell theory established in the 1830s by Matthias Schleiden & Theodor Schwann:

  • All organisms are composed of cells.

  • All cells arise from preexisting cells.

Grouping Cells

Morphological Grouping

• Two main categories:

  • Prokaryotes: No membrane-bound nucleus.

  • Eukaryotes: Have a membrane-bound nucleus.

Phylogenetic Grouping

• Three major domains of life:

  • Bacteria

  • Archaea

  • Eukarya

Prokaryotic Cells

Characteristics

• Prokaryotes lack a nucleus.

• Archaeal structure: Largely unknown.

• Bacterial structure:

  • Plasma membrane

  • Cytoplasm

  • Single circular chromosome (DNA)

  • Ribosomes for protein synthesis

  • Peptidoglycan cell wall

  • Polysaccharide capsule

Chromosome Structure

• Prokaryotes typically have one supercoiled circular chromosome located in the nucleoid region.

Eukaryotic Cells

Size and Complexity

• Typically larger than prokaryotes.

• Complexity arises from the compartmentalization into organelles:

  • Offers advantages like separation of incompatible chemical reactions and increased reaction efficiency.

Key Differences from Prokaryotes

• Eukaryotic chromosomes are enclosed within a nucleus.

• Greater cell size and presence of internal membranes.

• Eukaryotic cells possess a dynamic cytoskeleton.

Cell Types

Generalized Animal Cell

• Structures unique to animal cells include:

  • Centrioles

• Average size difference:

  • Prokaryotes: 10x smaller in diameter and 1000x smaller in volume compared to eukaryotic cells.

Generalized Plant Cell

• Unique structures in plant cells include:

  • Cell wall (cellulose)

  • Chloroplasts

Cytoplasmic Structure

Cytoplasm

• Region between the plasma membrane and nucleus, composed of 70-80% water (semi-solid). Contains:

  • Organelles

  • Cytoskeleton

  • Various biochemicals

Nucleus

• Structure

  • Double-membrane nuclear envelope

  • Contains a nucleolus

  • Nucleoplasm and nuclear pores for transport.

• Function

  • Information storage, RNA synthesis, and ribosome assembly.

Organelles in Eukaryotic Cells

Ribosomes

• Non-membranous, pivotal in protein synthesis, found attached to rough ER or free in cytosol.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes, processes and folds proteins.

• Smooth ER: Involved in lipid synthesis and detoxification, storing calcium ions.

Golgi Apparatus

• A series of stacked sacs that processes, sorts, and ships proteins from the rough ER.

Peroxisomes

• Organelles involved in oxidation reactions, break down fatty acids, and detoxify poisons.

Vacuoles

• Large structures primarily for storage in plant cells, help maintain turgor pressure.

Mitochondria

• Double-membraned organelles, the site of ATP production through cellular respiration.

Chloroplasts

• Found in plant cells, double membranes, facilitate photosynthesis by converting light energy to chemical energy.

Lysosomes

• Contain digestive enzymes for macromolecule digestion, mainly found in animal cells.

Cell Wall

• Present in plants, algae, and fungi. Provides mechanical support and protection.

Cytoskeleton

• Consists of protein fibers:

  • Maintains cell shape and stability, aids in movement, organizes organelles, and transports materials.

• Types of cytoskeletal elements:

  • Actin filaments (microfilaments): smallest, support cell shape and movement.

  • Intermediate filaments: provide structural support, shape, and position organelles.

  • Microtubules: large tubes that aid in vesicle transport and cell movement.

Cell Movement Structures

Flagella and Cilia

• Flagella: Longer, fewer in number, propel cells.

• Cilia: Shorter, more numerous, involved in movement and signaling.

Extracellular Layers

Extracellular Matrix (ECM)

• Provides structural support and varies in composition based on cell type.

Cell Junctions

• Types of junctions:

  • Anchoring Junctions: Strengthen connections between adjacent cells.

  • Tight Junctions: Prevent leakage of materials between cells.

  • Gap Junctions: Allow for direct communication between cells.

Summary

• Cells are dynamic, integrated structures that work in concert to perform essential functions. Understanding organelle structure and function is crucial for comprehending cellular processes.